Nishi, Keisuke (Deep Ocean Resources Development Co., Ltd. (DORD)) | Koizumi, Akira (Deep Ocean Resources Development Co., Ltd. (DORD)) | Tsune, Akira (Deep Ocean Resources Development Co., Ltd. (DORD)) | Tanaka, Soichiro (Deep Ocean Resources Development Co., Ltd. (DORD))
In this study, we have considered the relation between topographic features and the distribution of polymetallic nodules based on their sizes and chemical composition in the Japanese License Area, Clarion-Clipperton Fracture Zone, Central Pacific. It has been observed that small nodules are distributed on rather steep slopes with relatively low Ni + Cu content (average 2.0%), whereas larger nodules are distributed on gentle slopes with higher Ni + Cu content (ave. 2.5%). These indicate that the size and chemical composition of nodules are controlled by topographic features and growth processes.
Polymetallic nodules (referred to as the “nodules”) are found on the deep-sea floor in all the oceans. It is known that the Clarion- Clipperton Fracture Zone (CCZ) has the highest density of the nodules in the world's ocean (e.g., Morgan, 2000). As the nodules generally contain an average of 28.4% Mn, 0.2% Co, 1.3% Ni, and 1.1% Cu (e.g., Hein, 2016), they are expected to become potential deep-sea mineral resources in the future. To date, the International Seabed Authority (ISA) has singed exploration contracts with 17 Contractors, including Deep Ocean Resources Development Co., Ltd. (DORD), which holds an exploration right for nodules in the Japanese License Area (JLA). JLA is approximately 75,000 km2, which is considerably broader than that of other deep-sea mineral resources. To explore the vast area, it is essential not only to use wide-ranging probes together with autonomous underwater vehicles (AUVs) (e.g., Okazaki and Tsune, 2013), vessel-mounted acoustic survey systems, and deep- towed instruments (e.g., Sharma, 2010; Kuhn et al., 2011; Tsune and Okazaki, 2014; Schoening et al., 2016) such as a side scan sonar and deep-sea camera but also to conduct sampling of the nodules and sediment. Data on the size, weight, coverage, and abundance of the nodules obtained from the sampling surveys by using box corers, large corers, dredging, and others have been utilized for estimating the resource as well as understanding the origin of the nodules. In addition, the distribution characteristics of nodules in relation to topography has been analyzed from different approaches such as acoustic investigation and seafloor observations (e.g., Usui et al., 1987; Nishimura, 1992; Sharma and Kodagali, 1993). However, despite the vast area, publicly available data is limited, which results in inadequate understanding of an overall distribution and origin of the nodules. Especially, it is commonly known that the nodules of various sizes exist, but the relationship between their size and chemical composition is not yet clearly understood.